Fuel cell stacks wherein the cells in the stack use a phosphoric acid electrolyte along with oxygen and hydrogen reactants to produce electricity and water are well known in the prior art. Phosphoric acid fuel cell stacks of the prior art have been temperature controlled with a water coolant which passes through separate cooling passages in the stack and is exhausted from the stack cooling passages as a two phase water-steam mixture. The water and steam components of this two phase mixture are then separated whereupon the water can be used once more to control stack temperature and the steam is piped to a reformer to be reacted with a raw fuel, such as methane or the like, to produce the hydrogen fuel reactant for the stack. The temperature of the reformer is maintained at a proper level by an associated burner which has its own fuel supply. U.S. Pat. No. 4,344,849, granted Aug. 17, 1982, to A. P. Grasso et al. discloses a form of a fuel cell stack which operates generally in the aforesaid manner. A limiting factor in the operation of this type of fuel cell stack arises from the use of steam to operate the reformer. The temperature and thus the pressure of the steam generated is limited by the stack operating temperature. Current state-of-the-art is 405.degree. F. This limits stack operating pressure to the range of 120-170 psia. These relatively low operating pressures in turn limit the optimization of heat rate and operating efficiency, and also limit achievable cell power density with a resultant increase in cost. Additionally, as noted above, the use of steam to operate the reformer requires a reformer burner which, in turn, requires its own fuel source.
The system of this invention does not use steam to operate the reformer and therefore the stack can operate under higher internal pressures with resultant lower heat rates and attendant increased efficiency, as well as improved cell power density and lower cost. Additionally, the system of this invention does not require use of a reformer burner to maintain the proper operating temperatures in the reformer. Cooling of the stack in accordance with this invention is accomplished by cycling moist oxygen-depleted exhaust gas from the cathode side of the fuel cells back to the cooling passages of the stack. Before reaching the stack cooling passages, water is entrained in the recycled cathode exhaust gases in the form of a fog. The coolant in this invention thus is a mixture of water fog entrained in the moist oxygen-depleted cathode exhaust gases. This mixture is circulated through the cooling passages where the water droplets entrained in the mixture are vaporized so as to cool the cells in the stack. The exhaust from the cooling passages is thus a mixture of water vapor, oxygen and nitrogen. This contrasts to the steam and water coolant exhaust formed in the prior art. Since the only water present in the exhausted coolant is in the form of water vapor, the internal pressure of the stack can be maintained in the range of 200 psia to 600 psia, and preferably at 400 psia. This will be done with turbocompressors as will be pointed out in greater detail hereinafter. The exhausted coolant then has its temperature raised to an appropriate level whereupon it is piped into an autothermal reformer to react with a heated raw fuel, such as methane or the like. The presence of heated oxygen gas in the reformer renders the hydrogen-producing reaction autothermal so that the hydrogen gas produced in the reformer is at the same temperature as the incoming oxygen-nitrogen-water vapor reactant mixture without the need of an auxiliary reformer burner. The elevated operating pressure in the stack produce high stack cell power densities at lower heat rate, typically 400 watts/ft.sup.2 power density at a 7500 Btu/Kwhr heat rate. This compares to the steam reformer low pressure stack which will typically produce about 140 watts/ft.sup.2 power density at a heat rate of about 8300 Btu/Kwhr. Heat rate is defined as the Btu/hr of fuel input required per kilowatt of electricity produced.
It is therefore an object of this invention to provide an improved phosphoric acid fuel cell stack system which operates at higher efficiency levels, produces higher power densities and is more economical to operate.
It is an additional object of this invntion to provide a fuel cell stack system of the character described which can operate at higher internal pressures.
It is a further object of this invention to provide a fuel cell stack system of the character described which uses a gaseous mixture of oxygen, nitrogen, (or air) and water vapor as a reformer reactant.
It is yet another object of this invention to provide a fuel cell stack system of the character described wherein the reformer reaction is autothermal, requiring no auxiliary reformer burner for the reaction to be completed.
These and other objects and advantages of this invention will become more readily apparent from the following detailed description of a preferred embodiment thereof when taken in conjunction with the accompanying drawings.